The plasma display technology now makes it possible to achieve flat color panels of large size and with limited depth without any viewing angle constraints. The size of the displays may be much larger than the classical CPT picture tubes would have ever been allowed.
A plasma display panel utilizes a matrix array of discharge cells, which could only be “on” or “off”. Also unlike a CRT or LCD in which gray levels are expressed by analog control of the light emission, a PDP controls the gray levels by modulating the number of light pulses per frame (sustain pulses). The eye will integrate this time-modulation over a period corresponding to the eye time response.
Since the video amplitude determines the number of light pulses, occurring at a given frequency, more amplitude means more eye pulses and thus more “on” time. For this reason this kind of modulation is known as PWM, pulse width modulation. To establish a concept for this PWM, each frame will be decomposed in sub-periods called “sub-fields”. For producing the small light pulses, an electrical discharge will appear in a gas filled cell, called plasma and the produced UV radiation will excite a colored phosphor, which emits the light.
In order to select which cell should be lighted, a first selected operation called “addressing” will create a charge in the cell to be lighted. Each plasma cell can be considered as a capacitor, which keeps the charge for a long time. Afterwards, a general operation called “sustaining” applied during the lighting period will accelerate the charges in the cell, produce further charges and excite some of the charges in the cell. Only in the cells addressed during the first selected operation, this excitation of charges takes place and UV radiation is generated when the excited charges go back to their neutral state. The UV radiation excites the phosphorous for light emission. The discharge of the cell is made in a very short period and some of the charges in the cell remain. With the next sustain pulse, this charge is utilized again for the generation of UV radiation and the next light pulse will be produced. During the whole sustain period of each specific sub-field, the cell will be lighted in small pulses. At the end, an erase operation will remove all the charges to prepare a new cycle.
In static pictures, this time-modulation, repeats itself, with the base frequency equal to the frame frequency of the displayed video norm. As known from the CRT-technology, all light emission with base frequency of 50 Hz, introduces large area flicker, which can be eliminated by field repetition in 100 Hz CRT TV receivers. Contrarily to the CRTs, where the duty cycle of light emission is very short, the duty cycle of light emission in PDPs is around 50% for middle gray. Also, plasma display panels are operated in 50 Hz or 60 Hz progressive scan mode, i.e. a full frame is displayed in a 20 ms raster. Both of these reduces the amplitude of the 50 Hz or 60 Hz frequency component in the spectrum, and thus large area flicker artifact, but due to the larger size of PDPs, with a larger viewing angle, even a reduced large area flicker becomes objectionable in terms of picture quality. And this is also true for 60 Hz video norms. The present trend of increasing size and brightness of PDPs, will also contribute to aggravate this problem in the future.
A previous European patent application of the applicant addresses this problematic. Reference is made to EP document EP 0 982 708 A1. In this patent application it is proposed a grouping of the sub-fields in a frame period in two consecutive groups and a specific type of sub-field coding, which makes a distribution of the active sub-field periods equally over the two sub-field groups. With this solution practically a doubling of the frame repetition rate is accomplished. The 50 Hz or 60 Hz frequency component is substantially reduced and the image quality is improved. With the use of a sub-field organization where in both groups the most significant sub-field weights are identical the symmetrical distribution of light pulses over the two sub-field groups was relatively easy to achieve. Only small differences in luminance values respectively light pulse numbers needed to be tolerated. But such small differences did not introduce significant large area flicker.
Meanwhile, a new type of sub-field coding has been developed, which is called “incremental sub-field coding”. In this type of sub-field coding, there are only some basic sub-field code words used for the gray scale portrayal rendition. This means, that in the case of 8 Bit video data, there are not 256 different sub-field code words for the possible video levels, but instead only a few characteristic sub-field code words for some distinct video levels and the remaining video levels are rendered by some optimized dithering or error diffusion techniques. The specialty of the incremental code is that in each case there is never one sub-field inactivated between two consecutive activated sub-fields and vice versa. With this characteristic the incremental code has the advantage, that the false contour effect is no longer a problem due to the fact that sub-field code words for similar video levels cannot deviate at various bit positions. The structure of such sub-field code words is nearly identical, except for only one sub-field entry. In order to achieve a compensation for the gamma pre-corrected input video signal, it is advantageous for this incremental code, that the sub-fields weights follow a specific behavior, in particular it should follow a gamma function. This means, that the sub-field weights of the sub-fields in a sub-field organization for a frame period increase step-by-step. As a result, there are no sub-fields in the sub-field organization, which have identical sub-field weight. The incremental coding method is disclosed for example in the European patent application EP-A-0 952 569.
With such an incremental code, it is not so easy, to apply the above-explained concept of sub-field grouping and symmetrically distributing the activated sub-field periods over both groups for large area flicker reduction.